The use of powder coating processes and materials for sealing and coloring of finished surfaces has become increasingly popular in recent years. Powder coating offers several advantages over conventional painting and dipping techniques in which a solvent-borne paint, lacquer, epoxy or other coating is applied to a surface. Unlike these solvent-based coatings, powder coating is a dry process in which no volatile solvent is utilized. Thus, no potentially harmful solvents are released into the environment or work place by powder coating. Additionally, improved control over the application of coating materials is possible in powder coating, since the coating materials are attracted to the object being coated. The nature of this attraction is explained in detail below. Finally, solvent-based coating systems waste substantial quantities of material due to overspray and adhesion of coatings to paint cans, spraying equipment and other vessels. Conversely, powder coating materials are almost entirely transferred from storage vessel to the object. Furthermore, oversprayed materials in powder coating can, potentially, be retrieved for reuse with proper handling.
Powder coating is an electrostatic process that generally entails the charging of particulate coating materials. These materials typically comprise thermoset plastic compounds that fuse together upon application of heat. The electrically charged particles of material are transferred from the end of a gun to the object to be coated. The object is typically a metal object that is grounded so that it is neutrally charged. The charged particles adhere to the surface of the object in a relatively even layer that is between 1 to approximately 4 or 5 thousandths of an inch (mils) in thickness. Once coated, the loose particles on the surface of the object must be cured to permanently adhere to the surface of the object. The object is transferred to an oven, where it is baked. The baking process causes the particles to fuse to each other and to the underlying surface. The resulting coating is exceptionally strong and resilient and resists most forms of environmental and chemical corrosion.
Powder coating can produce a variety of surface finishes by this process, including shiny, dull, metallic or reflective surfaces. Thus, powder coating is a highly versatile process that can be applied to a myriad of diverse products ranging from aerospace components to chandelier parts.
Two basic methods, and their corresponding devices, are currently utilized for applying powder coating materials. The first process, known as the "Corona" process, which is detailed in FIG. 1 (described below), involves the use of an electron gun tip that generates a corona of electrons that negatively charge the powder coat particles by attaching electrons to surrounding air molecules. The negatively-charged particles are directed to the surface of the object and adhere in layers to the object. The particles are attracted to the metallic surface and tend to build up on layers upon the surface over substantially the entire surface area of the object. As the particles build, the distance from the metallic surface to the particles becomes greater causing the attractive forces between the upper layer particles and the metallic object surface to decrease. When the attractive forces are outweighed by the interparticle repulsive forces, further build-up is typically prevented. At this point, the surface will not accept further particles of powder particles and any additional particles tend to fall loosely from the surface.
An alternate approach to powder coating entails the use of a tubular gun having an electric field along its inner surface that strips electrons from the particles. As the particles move down the gun, they become positively-charged by a process known as triboelectric charging. The exiting particles, thus, carry a positive charge, as detailed in FIG. 3 (described further below). The positively-charged particles adhere to the surface of the object in a manner similar to the negatively-charged particles of the corona process described above. Again, once a sufficient number of positively-charged powder coated particles are layered upon the object's surface, the forces of attraction become outweighed by the interparticle repulsive forces and, thus, further particles of powder coat fall loosely from the surface.
FIG. 1 details a schematic flow diagram of a powder coating apparatus and process according to this invention. The process illustrated employs a corona process powder coat gun 30 that receives high voltage electricity via a cable 32, interconnected with a voltage source and controller 34.
The gun 30 is further detailed in FIG. 2. It generates a corona 33 of electrons through which a spray 35 of particles passes. The corona 33 is generated between concentric electrodes 37 and 39. Voltage is provided by a cable 41 and pressurized powder coating particles are provided at an inlet 43. The flow of particles through the corona 33 is adjustable using the rotatable tip 45.
The controller 34 and gun 30, according to this embodiment, can comprise a Nordson Corporation 100 Plus.RTM. Power Unit and a corresponding Versa-Spray.TM. Cable-Feed Manual Spray Gun. The Nordson 100 Plus.RTM. Power Unit includes a voltage multiplier that is adjustable between 30 and 100 KVDC (30,000-100,000 volts) to generate a corona that electrostatically charges the powder particles. The control at 34 includes a power switch 36 and a voltage control 38 that regulates voltage in the cable 32 performing the corona at the tip 40 of the gun 30. A display 42 indicates the voltage.
The controller 34 also includes an inlet 44 for receiving compressed air from an air line 46 that is interconnected with a compressor 48. The controller 34 includes an atomizing air control knob 50 and a flow rate air control knob 52 with corresponding output meters 54 and 56 that control outgoing air flow to a pair of lines 58 and 60. The lines 58 and 60 are connected to a powder pump module 62 that receives powder particles via a short interconnection 62 from a powder particle feed hopper 66. Particles are drawn into the pump 62 by action of the flow rate air line 60 and are dispersed by the atomizing air line 58. Pressurized dispersed particles are delivered to the gun 30 via an output line 64. The particles exit the gun 30 by action of the trigger mechanism 67 (see FIG. 2) which opens a valve (not shown) in the gun allowing the pressurized particles to exit the gun tip 40. As noted above, a corona formed at the tip 40 charges the particles and also generates a field between the tip 40 and the object 68. As illustrated, a plurality of field lines 70 are defined between the tip 40 and the object 68 and the particles 72 travel generally along the field lines 70. Note also that the object 68 is interconnected with the ground 76 to neutralize any surface charge held by the object 68.
Subsequent to coating of the object 68 with a sufficient quantity of powder particles 72 to obtain an even surface coat, the object 68 is transferred to a heating station that can comprise an oven adapted to receive the particular object shape. A heating element 80 delivers heat to the surface of the object 68 to cause the particles to melt and fuse to each other performing an even and uniform surface across the object 68. Heating typically occurs for a period of time from 10 to 30 minutes at temperatures of approximately 350.degree. to 400.degree. F. Many powders specify exact temperatures and time durations for the curing process.
Subsequent to heating, the object is allowed to cool to room temperature, either within the oven or at a cooling location 82 remote from the over. The finished object can then be handled normally.
As discussed above, alternate approach to applying particles to an object 68 known as the triboelectric process is illustrated in FIG. 3. This process utilizes a feed pump 162 and compressor 148 having air outlet line 146, flow rate and atomization air pressure lines 160 and 158, respectively, similar to those used in the corona process of FIG. 1. Similarly, a controller 134 is utilized to control the powder atomization and flow rate via controls 150 and 152, respectively. Values for atomization and flow rate pressure are read on corresponding meters 154 and 156, respectively. The pressurized particles are transferred down the outlet feed line 164 to the triboelectric gun 130. The gun 130 is, likewise, fed high voltage electricity from the controller 134 via a cable 132.
The gun 130 comprises an elongated metallic tube 190 that generates a substantial electric field within its inner diameter 192. The gun 130 is operated to induce particle flow by the trigger 167. As particles move down the tube 190 under pressure, they strike each other and the walls of the inner diameter 192 of the tube 190 becoming "triboelectrically-charged". In other words, the particles are stripped of electrons through action of the field. Thus, the particles exit the tip 140 of the tube 190 in a positively-charged state. The charged particles are attracted to the object 68 in a manner similarly to the negatively-charged corona particles of FIG. 1. The object 68 is connected to ground 176 as in the corona process of FIG. 1.
The triboelectric process differs from the corona process, generally, in that no electric fields are generated between the tip 140 and the object 68. Thus, the particles tend to strike the object in a more-dispersed manner. However, the resulting coating of the object is of somewhat similar quality and differs from the corona process primarily in that the surface particles carry a positive charge rather than a negative charge. Curing of the triboelectrically-charged particles occurs in a substantially similar manner to curing in the corona process. A heating element 180 provides heat in a range of approximately 350.degree.-450.degree. F. for a time period of 10 to 30 minutes to the object 68, which is then generally allowed to cool at a cooling location 82, either within, or remote form the heating area.
Both the corona and triboelectric process are typically carried out within an appropriate spray booth, having adequate ventilation and protection from loose flying particles.
A current disadvantage of the application of powder coatings using both the corona and triboelectric processes is that surface thicknesses remain somewhat variable. In fact, a surface on a typical object can vary in thickness by up to two to three thousandths of an inch between different areas on the surface. Such an uneven surface coating can be undesirable, particularly where reflective surfaces are used or where a precision appearance is desirable.
It is, therefore, an object of the present invention to provide an improved method and apparatus for applying powder coatings to surfaces of objects with better control of the thickness of the coating layer. This invention should also provide a method and apparatus that generates a more even coating layer thickness over the object with a minimum of surface imperfections in the coating layer.
This method and apparatus should be implemented with a minimum of additional equipment and should be adapted for use by ordinarily skilled technicians in the field.